Corrosion and Carburization Behaviour of Ni-Cr-Mo-Nb Superalloys in a High Temperature Supercritical-CO2 Environment

Abstract

Two Ni-Cr-Mo-Nb superalloys (Alloy 625 and Alloy 718) were corroded in high temperature supercritical-CO2 (S-CO2) at 700 degrees C (20 MPa) for 500 h and compared in terms of oxidation and carburization behavior. A continuous chromia (Cr2O3) layer was formed on the surface of Alloy 625, whereas Ni- and Fe-rich oxide nodules were also formed with chromia on Alloy 718. Meanwhile, the extent of carburization by formation of an amorphous C layer at the oxide/matrix interface was comparatively low for Alloy 625. This difference did not seem to stem from oxide type or underlying microstructure, and was thought to be associated with oxide properties. In terms of mechanical properties, only Alloy 625 exhibited decrease in ductility after exposure to S-CO2. This was ascribed to the microstructural evolution of the alloys during the high temperature exposure.